Garbage Collection question

[Wu Wen]

50. Given:
3. public class Fiji {
4.      static Fiji base;
5.      Fiji f;
6. public static void main(String[] args) {
7.     new Fiji().go();
8.     // do more stuff
9. }
10. void go() {
11.    Fiji f1 = new Fiji();
12.    base = f1;
13.    Fiji f2 = new Fiji();
14.    f1.f = f2;
15.    Fiji f3 = f1.f;
16.    f2.f = f1;
17.    base = null; f1 = null; f2 = null;
18.    // do stuff
19. } }

Which are true? (Choose all that apply.)
A. At line 8, one object is eligible for garbage collection.
B. At line 8, two objects are eligible for garbage collection.
C. At line 8, three objects are eligible for garbage collection.
D. At line 18, 0 objects are eligible for garbage collection.
E. At line 18, two objects are eligible for garbage collection.
F. At line 18, three objects are eligible for garbage collection.

The right answers are C, D. Anyone can explain me the steps in detail? Thanks a lot

[Henry Wong]

Please QuoteYourSources

[Wu Wen]

Henry Wong wrote:
Please QuoteYourSources

The question comes from OCP Java SE 6 Programmer Practice Exams Page 181

[Sebanti Sanyal]

Inside method go(),two objects are created. f3 refers tothe object originally referred by f2. That particular object has a member variable f,which again refers to the object initially referenced by f1. Therefore, none of the two are eligible for GC at line 18. If we also had f3=null;, the two objects would still had referenced each other but none of them could be accessed externally and hence eligible for GC.
In main(),one object is created on which go() was invoked.That object was created on the go,and there is no reference associated with it.Hence, it is definitely eligible for GC at line 8.The two objects created inside go() were accessible only with a local variable f3,which does not live anymore after go() returns. These two can also be garbage collected at line 8.

[Wu Wen]

Sebanti Sanyal wrote:Inside method go(),two objects are created. f3 refers tothe object originally referred by f2. That particular object has a member variable f,which again refers to the object initially referenced by f1. Therefore, none of the two are eligible for GC at line 18. If we also had f3=null;, the two objects would still had referenced each other but none of them could be accessed externally and hence eligible for GC.
In main(),one object is created on which go() was invoked.That object was created on the go,and there is no reference associated with it.Hence, it is definitely eligible for GC at line 8.The two objects created inside go() were accessible only with a local variable f3,which does not live anymore after go() returns. These two can also be garbage collected at line 8.

Thank your for the explanation it is very clear.

If I remove base=null, what will be the result? I'm a little confused because base is static. Thanks

[Sebanti Sanyal]

If I remove base=null, what will be the result? I'm a little confused because base is static. Thanks

Answer will be A and D. The objects created inside go() can still be referenced through base at line 8.

[Selena Klasnja]

Wu Wen wrote:

Sebanti Sanyal wrote:Inside method go(),two objects are created. f3 refers tothe object originally referred by f2. That particular object has a member variable f,which again refers to the object initially referenced by f1. Therefore, none of the two are eligible for GC at line 18. If we also had f3=null;, the two objects would still had referenced each other but none of them could be accessed externally and hence eligible for GC.
In main(),one object is created on which go() was invoked.That object was created on the go,and there is no reference associated with it.Hence, it is definitely eligible for GC at line 8.The two objects created inside go() were accessible only with a local variable f3,which does not live anymore after go() returns. These two can also be garbage collected at line 8.

I think it is a good explanation but only if f3 doesn't play any further role. For example:
What would be the scenario if we instead of doStuff at line 18 have something like this:

new HoldFiji(f3);

class HoldFiji
{
    static Fiji myFiji;
    HoldFiji(Fiji fiji)
    {
        myFiji = fiji;
    }
}